The day I visited 30 St Mary Axe, the City of London was smothered in a Dickensian mist. Norman Foster's 180m tour de force was all but invisible. Its spectacular top-floor bar offered panoramic views not of a great carpet of buildings patched with parks and threaded with rivers, but of nothing at all. Nothing, that is, save the eerie sight of spidermen absailing the outside of the great steel-and-glass sheath, fixing its 7,500 windows permanently into place.

The poor weather was useful in that it concentrated my mind on the building itself rather than any mesmerising views. This seemed right. For this is an extraordinary building, one that every Londoner, every visitor to the capital, should want to get to know.

The curious have watched the skywards odyssey of this radical skyscraper since construction began in 2001 and have known about it for at least five years. It rises from the site of the former Baltic Exchange building bombed by the IRA in 1992. Last December, the skeletal structure was decorated like a Christmas tree. It was visible night and day from the least likely corners of the capital, above rooftops, scrapyards, marshes and motorways. Like the dome of St Paul's Cathedral, Canary Wharf and the future London Bridge Tower, designed by Renzo Piano, 30 St Mary Axe will define the London skyline for generations to come.

It already has a nickname. A dumb one: the Gherkin. I am not sure where this name originated, but it is hopelessly inappropriate, even if, like the "wobbly bridge", it is likely to stick. Gherkins do not look like this. Nor is 30 St Mary Axe, in any accepted understanding of the word, "organic". If it resembles anything, it is one of the great dirigibles of the 1930s, or one of Wernher von Braun's early space rockets. Tethered firmly to the ground, this sleek and sensational machine for making money, this "towering innuendo" commissioned and owned by financial services group Swiss Re, will be home to up to 4,000 workers. It is certainly a striking design, but will 30 St Mary Axe offer better conditions than workers have been used to? Will it be more than yet another filing cabinet in the sky?

Client and architect, working with the innovative service engineers BDSP, have made great claims for the building's environmental efficiency. One of the less measurable ways in which the tower can be considered environmentally friendly is the manner in which, close up, it appears to be much smaller, or at least, much lower, than it is. In fact, it appears no higher than GMW's Commercial Union building (which is some 70m shorter), an illusion created by its airship profile, curving upwards and inwards towards a nose cone invisible from the square below.

Not only does the building's circular form make it appear much less bulky than it is, it generates considerably less wind at street level than many right-angled towers. This has been proven in wind-tunnel tests on a model of the building. The circular form also offers a generous public plaza at the base, while an arcade around the tower promises a number of useful shops.

The real environmental achievement here, however, is the internal design. What you see from the plaza and, in fact, from all corners of London, is a great sheath of steel, aluminium and glass. This is the building's skin. Just behind this is the great steel structure of the building, hidden on dull days, clearly visible when the sun shines. This structure, devised by the engineers Arup, is a diagonal cage very much like the skeleton of Barnes Wallis's second world war bombers. Intriguingly, this structure compresses during the day as the building loads up with people, and stretches in the evening as it empties.

Spiralling up through the internal structure is a sequence of atriums. They are interrupted every six floors so that the updraft of air through the building does not become too strong. These atriums achieve many things: they let light deep into the plan and allow diagonal views up and down through the building. But the main advantage is that their corkscrew shape creates different air pressures, ensuring that fresh air is sucked up through the building as well as through the office floors.

In addition, the windows on the skin of the building can be opened in mild weather to allow air to flow in and out. Roller blinds set between the steel skeleton and glass skin control glare and reduce heat. Heating, lighting and air-conditioning bills will be low compared with most towers and, with daylight reaching desks at the core of the building, workers should feel well off.

The luckiest will be able to reach the crown, cap or nose cone of the tower, where a restaurant and bar will satisfy anyone with a craving for James Bond glamour. The bar, reached by a spiral stair or glass lift from the 70-seat restaurant, looks very much like a Ken Adam film set, the lair of a villain with designs to take over the world. It is quite breathtaking. Sadly, it and its master-of-the-universe views will be for Swiss Re employees and their guests only.

This is a fine skyscraper and one of the shapes of things to come in the design of city-centre offices. It is not, however, perfect. Plans for planted meeting rooms projecting into the six-storey atriums and visible from floors above have been dropped. Nor is the tower quite as "green" as Foster's earlier 299m Commerzbank, Frankfurt, completed in 1997: the tallest and, to date, probably the most advanced office tower in Europe.

The building is also, perhaps, a little too smooth for its own good. Detail is not so much pared down as polished away, as if the architects had tried to make it as sheer as an airship, without the touches that delight hand and eye. Its entrance is also problematic; carving an appropriate doorway into the base of this office rocket was always going to be difficult, and the result seems a little crude, as if someone had opened up a cavity with a giant tin-opener and then tried a little too hastily to cover their handiwork. It would, perhaps, have been better to have created more than one underground entrance, which might have swept those working here from the edges of the plaza up into the aluminium-lined entrance lobby, leaving the skin of the building unbroken. This a trick Oscar Niemeyer performed neatly, and dramatically with the entrance of his "crown-of-thorns" cathedral in Brasilia. Doubtless there are practical, not to mention security reasons why this could not have been.

This might sound like nitpicking. But it would be good to see the hugely successful Foster office loosening up its super-streamlined approach to design and allowing a little more sensuality in with the air. Foster's own 1980s masterpiece, the headquarters of the Hong Kong Shanghai Bank, was, with its animated structure, covered public plaza and rich detailing, a good precedent. It was also a famously expensive building and, while Swiss Re has been a thoughtful and generous client, they have kept costs down.

The result is a fascinating building that undoubtedly raises the standard of city-centre office design, but one that should be regarded as a kind of work-in-progress: a staging post on the way to a more responsible and attractive form of skyscraper. In a Britain of largely cynical, fast-buck, skin-deep, government-approved new architecture, this is one new building - not a gherkin - that deserves to be relished.